The persistence of influenza virus genomes is to be examined after acute infections of mice and of cell cultures. Initiating infections will be established with either infectious virus or with defective-interfering particles. At intervals thereafter challenge infections will be accomplished with heterotypic virus and a search made for recombinant viruses from these superinfected tissues. It is postulated that persistence would represent suppression of virus activity by cell- or virus-determined factors. Superinfection would then permit at least partial expression of the original, persistent infection either by providing genomes for recombination or for helper function. If persistence can be demonstrated the site of genoma persistence will be explored. Studies with syngeneic lymphocytes and with macrophage cultures will be undertaken to determine their role in this phenomenon. If, as in the case of many RNA viruses, persistent infection of lymphocytes can be established experiments will be carried out to artifically introduce inapparent persistent infections into mice and to then examine the effect of superinfection with heterotypic infection both as to activation of the virus genome, and as to the host immune responses to the infection.